dax: Avoid ABI change in 4.17.6

dax: Avoid ABI change in 4.17.6

The return type and first parameter type for bdev_dax_supported() and
__bdev_dax_supported() were changed by commits ba23cba9b3bd "fs: allow
per-device dax status checking for filesystems" and 80660f20252d "dax:
change bdev_dax_supported() to support boolean returns".

Avoid an ABI break by renaming the new version of
__bdev_dax_supported() and reintroducing the old version as a wrapper
for it.  Add a #define so that the old version is hidden from the API,
i.e. newly built modules must use the new API.

Gbp-Pq: Topic debian
Gbp-Pq: Name dax-avoid-abi-change-in-4.17.6.patch